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-Astroprof

The remote location delayed word reaching scientists in major cities. The remote location also meant that travel to the site was an expedition rather than just a trip. Considerable planning was needed, as well as gathering of supplies. Before scientific expeditions could make it into the area, the world fell into war. The Great War, now known as World War I, pretty much kept everyone occupied for a number of years. After the war, Russia was deep into the throes of revolution. So, it was over two decades before scientists made it into the area. What they saw shook the world. An entire forest was devastated by the explosion.

Right away, speculation began to run rampant. Nobody had seen something like this. But, just a few years later, that changed. During World War II, weapons scientists began to realize that for very large bombs, the overpressure can do more damage than the immediate explosive fireball. And, to maximize the coverage of that overpressure, the bomb should be detonated above the ground. The blast damage from the atomic bombs dropped on Japan at the end of the war displayed similarities to the Tungaska blast pattern, only the Tunguska blast was much, much larger.  Soon, a favored hypothesis was that the blast was caused by some sort of air burst. But, an air burst of what?

The Solar System is a shooting gallery. There are a lot of things flying around out there. Most of these things are small, so when they run into Earth, they simply appear as meteors (shooting stars). A few, though, survive passage through Earth’s atmosphere to strike the ground. These are meteorites. The larger the meteorite, the bigger the explosion when it hits the ground, and the bigger the crater. Earth has plenty of craters.

But, the Tunguska event shows signs of an atmospheric explosion, not a crater. There have been numerous attempts to find a crater, but so far all have been fruitless. At present, there are still a few claims that have yet to be evaluated by the scientific community, but most today feel that there is no crater. So, how could something so big hit Earth and not leave a crater? Well, it obviously had to be something that did not make it to the ground. So, what could that be?

One of the early contenders was that it may have been something that would not survive the high temperature and pressure of passing through the Earth’s atmosphere. A comet was suggested as fitting the bill. After all, comets are icy bodies, so they would tend to vaporize during entry into Earth’s atmosphere. Of course, it would have to be a smaller body than most comets, so perhaps it was a piece of a comet that had broken off. An likely parent body was even postulated: Encke’s Comet. Comet Encke was known to shed pieces now and then. And, Encke’s Comet comes close to Earth. In fact, in June, Earth is quite near the comet’s orbit, passing through a swarm of debris shed by the comet. This debris gives us the Beta Taurid Meteors, which peak in late June and early July. Furthermore, the bodies approach Earth from the daylight side of the planet, not unlike the object that created the blast. However, this hypothesis has gradually fallen into disfavor.

The top hypothesis today is that a stony asteroid was the progenitor of the Tunguska blast.  But, how can a huge chunk of rock not make it through the atmosphere?  Well, remember the asteroid Itokawa.   That is an example of a rocky asteroid that is not a solid chunk of rock.  It is at best a pile of rubble.  Such a body would hit the atmosphere moving at dozens of kilometers per second and shatter into billions and billions of pieces from the shock of the sudden deceleration.  Those pieces would separate, the debris would pancake, and the air in front of the body would be compressed and heated into a great fireball.  The energy released by the ensuing explosion would be huge.  For a body of only tens of meters across, the resulting explosion could easily be equivalent to that of a hydrogen bomb.  Even for a fairly solid rock, the stress of hitting the atmosphere would be an awful lot to stand.  For rocky bodies, the tiny ones burn up.  The small ones make it to the ground as meteorites.  The medium sized ones blow up in the atmosphere.  The large ones, miles across, would probably make it to the ground.   Favoring the asteroid hypothesis is dust found at the impact site consistent with the composition of asteroids.

So, both the asteroid and comet hypotheses are still alive, but the scientific community is leaning heavily towards an asteroid, based on the evidence currently available.  Also, Earth crossing asteroids in that size range are very common.  Comets or comet fragments of the right size are quite rare by comparison.

But, just how big was the explosion?  For many years, I had heard estimates of about the equivalent of 25 megatons of TNT.  But, in recent years, the estimate had dropped considerably, with about 12 MT being about average.  There have been suggestions that the trees of the area were easier to knock over than had been thought, and so the blast damage was overestimated.   I have heard estimates of blast strength as low as about 3 MT.  That is about what you’d expect from a hydrogen bomb.  This happened a century ago.  But, there is a reason to try to nail down the size of the blast beyond simple curiosity.  The smaller the blast, the smaller the body that caused it.  And, there are a lot more small bodies flying around than large ones.  So, knowing the size of the body responsible for the blast gives us an idea of how likely it is happen again anytime soon.

Estimating the size of the body causing the explosion is difficult.  For one thing, we don’t know how big the blast really was.  For another, we don’t know how fast the impacting body was moving.  Recently, we’ve been able to compute the atmospheric effects far better, and that suggests a much smaller body may have been responsible than had generally been assumed.  If so, then the risk of another Tunguska event goes up.  The smallest bodies that I’ve seen proposed are believed to strike Earth perhaps once every couple hundred years.  Now, that doesn’t mean that we are safe for another hundred years.  Ask the people in Iowa that have had their second hundred year flood in under two decades.  A hundred year flood simply means a 1% chance of flooding each year.  A once every couple hundred year chance of impact really means is that there is a 0.5% chance of impact each year.  Of course, I’ve heard other estimates that were far more comforting, such as a chance of impact once every thousand years or so (about a 0.1% per year).

Over the years, of course, there have been far wilder suggestions of what caused the impact, ranging from a miniature black hole to a chunk of antimatter.  And, there have been suggestions of non-natural causes, too, such as a crashing flying saucer or a scientific experiment gone awry.  But, the simplest and far most likely scenario is of an impact by an asteroid or comet.

-Astroprof

Images courtesy Wikimedia Commons

When you look up information about planets, one of the bits of data given is the albedo of the planet. Albedo is one of the vocabulary words that introductory astronomy students have to learn. According to the textbook that we are using, Mars has an albedo of 0.15, Jupiter has an albedo of 0.44, and Venus has an albedo of 0.59. So, what is albedo? What do these numbers mean?

Put very simply, albedo is a measure of the reflectivity of a body. You compute the albedo by dividing the amount of reflected light by the amount of incident light. So, an albedo of 0.25 means that a body reflects 25% of the light that shines on it. Unless otherwise stated, albedo normally refers to visual light. Rocky bodies, such as Mercury or the Moon, have low albedos. They are gray, and they absorb more light than they reflect. Icy bodies, such as Pluto, reflect a lot of light, so their albedos are high. Venus is covered in clouds that are very reflective, so it has an albedo greater than 0.5. That means that it reflects more light than it absorbs.

We talk about the albedo of planets, comet nuclei, moons, and asteroids. Another related term is absolute magnitude. In astronomy, magnitude is a measure of how bright an object appears. My stellar astronomy students know the term absolute magnitude as being how bright a star would appear if it were located at a distance of 10 parsecs (32.6 light years) away from us.  It is a way of differentiating how bright an object really is from how bright it appears.  Such a thing would also be useful for planets, asteroids, and comets.  It is a way to directly compare them to one another.  A larger body will appear brighter, simply because it reflects more light because of its size.  A smaller one, though, could appear just as bright if it were more reflective and had a higher albedo.  So, we can define something analogous to stellar absolute magnitude for objects within the solar system.  Unfortunately, to the consternation of hosts of astronomy students, the term used is the same term:  absolute magnitude.  Of course, when we are talking about the absolute magnitude of a planet or asteroid, we definitely do not mean how bright it appears if it were a distance of ten parsecs away.    Instead, this planetary absolute magnitude is basically how bright a body would appear as seen from the Sun if it were at a distance of 1 AU from the Sun (that is the distance that Earth is from the Sun).  The absolute magnitude of a body in this system can be computed using the equation:

The H stands for the absolute magnitude (to avoid confusing it further with the stellar absolute magnitude, usually referred to as M in equations).  D is the diameter of the body in kilometers.  Naturally, for irregularly shaped bodies, it would be the effective average diameter.  A in this equation is the albedo.  There are other factors, of course, that I am not considering.  Some substances reflect light differently at different angles of incidence.  And, of course, some objects reflect different colors of light differently.  But, this is a pretty good approximation.  It is as far as we get in the introductory classes.

- Astroprof

-Astroprof

According to a JPL press release, the Phoenix lander on Mars recently had an issue with its flash memory.  You can read more about the incident on Emily Lakdawalla’s blog posting about it.  Flash memory is a nonvolatile memory such as that used in memory sticks.  The RAM on board the spacecraft loses data when it is powered down for the night.  But, the flash memory holds onto the data.  Flash memory is very durable, so it is normally a safe way to store data.  However, Tuesday night, Phoenix lost some data when it powered down.  Apparently what happened was that there was not enough memory to hold onto all of the data that was trying to be stored.  The spacecraft stores not just science data, but also information about spacecraft operations, too.  Maintaining the health of the spacecraft obviously has high priority.  After all, if you lose the spacecraft, then you lose all future science data.  Normally, there isn’t all that much spacecraft data that needs to be stored, so there is plenty of room on the flash memory for science information.  Apparently, though, something happened Tuesday to cause the spacecraft to store a great deal more spacecraft data than normal.  That didn’t leave room for all of the science data.  So, when Phoenix powered down, the science data was lost.

The data that was lost was, for the most part, not all that critical.  It was mostly images.  And, of course the cameras can take more pictures, so that is not a big deal.  Apparently, the robotic arm did do a bit of digging, and not all of the images taken of the ground before the digging got transmitted to Earth, so that information is lost, but that is likely not a major deal.  What is worrisome, though, is why the spacecraft had so much housekeeping data that it displaced science data.  The Phoenix team is working on that, though.  For now, the plan is to make sure that the science data is transmitted back to Earth, if at all possible, before powering down the spacecraft.  In the meantime, they are trying to make sure that this won’t happen again, even if they don’t transmit all the data back home.

When I heard about a problem with Phoenix’s flash memory, it reminded me of a far more serious problem that the Mars rover Spirit had with its flash memory.  Nearly four and a half years ago, Spirit was having a problem where it rebooted several times per day.  That problem was solved by reformatting the flash memory.  The problem with Phoenix sounds different.  Hopefully, they’ll quickly figure out the problem and be able to correct it.  They don’t seem too worried about the situation, though, so that is good.

 -Astroprof

Image courtesy NASA, JPL

Since ancient times, people have looked at the sky and observed that some “stars” appeared to wander from constellation to constellation. These wandering “stars” were called planets. Jupiter is one of these planets. It takes about 12 years to move around the sky. But, people have also observed that night after night, the entire sky shows a subtle shift to the west when observed at the same time of night, about 1 degree per day. This is due to Earth’s motion around the Sun.

Since Earth moves around the Sun faster than Jupiter, it is no wonder that we left it behind. It was prominent in the sky last summer. But, Earth moved on. Eventually, by winter, the planet was hidden behind the Sun. (For my readers in the southern hemisphere, reverse the terms “summer” and “winter”!) But, you might wonder, “Where is Jupiter?” You might say to yourself, “I saw it about sunset last year at this time. Where is it now?” After all, it’s been a year, and Earth is back to where it started. So, what has happened to Jupiter? Well, quite simply, it moved.

Remember, planets move around the Sun. Jupiter is a planet that is farther from the Sun than Earth. So, that means that once in a while, Earth gets between Jupiter and the Sun. When that happens, Jupiter is said to be in opposition. That means that it appears opposite the Sun as seen from Earth. Jupiter was last at opposition on June 6, 2007. On that date, Jupiter rose at about sunset and set about sunrise. It was up all evening. It was visible as soon as the sky got dark enough to see it during twilight.

But, one year later was June 5 (Remember that this year was leap year, so one orbit of the Earth was 365.26 days later, and that was June 5). But, Jupiter was nowhere to be seen. You had to wait a bit over two hours after sunset for Jupiter to rise. What happened? Well, remember, Jupiter moves, too. Since it takes Jupiter almost 12 years (11.86 years, actually) to orbit the Sun, in the year that it took Earth to go around the Sun, Jupiter had moved about 1/12 of the way around its orbit, so it was no longer opposite the Sun from Earth in early June.

One orbit of Earth around the Sun did not result in another opposition for Jupiter (or any other planet). So, Earth must move a bit farther around its orbit for it to line up again so that Earth is between the planet and the Sun. Thus, the next opposition of Jupiter will not occur until July 9, 2008. So, oppositions of Jupiter occur at intervals of just over 13 months. This time interval, from opposition to opposition, is called the synodic period of a planet. Jupiter’s synodic period is about 399 days.

Since Jupiter will be rising shortly after sunset in early July, and it will be progressively rising earlier as the month wears along, then it will be a favorite target for amateur astronomers all month long, and for the next several months, too. July will be particularly good, though, because we will be closest to Jupiter at that time, so it will look slightly larger in the telescope than normal. But, because Jupiter is so far from us to start with, this effect is far less noticeable than it is with Mars.

But, you don’t have to wait until July to see Jupiter. You can still see it now. You’ll just have to stay up a bit later. For my part of Texas, Jupiter is currently rising just after 10pm. At the top of this posting is a view of what the sky would look like at almost 11:30 at night on June 19 facing to the southeast. I picked that night, two nights from now, because the Moon will be very close to Jupiter then. It will be just a little to the west of Jupiter. The following night, it will be just a little to the east of Jupiter. That will make it appear a little down and to the left of Jupiter as seen from Texas. In fact, on June 20, if you are in the right part of the world, the Moon will be passing Jupiter at a distance of only about 4 times the Moon’s apparent diameter. But, that happens just after dawn on that date, as seen from here. Observers in the Pacific, Australia, or Asia will see that happen, though.

-Astroprof

Skyview produced using Stellarium software

It isn’t every day that a new mineral is discovered. But, I recently read a NASA press release about a new mineral discovered in some comet dust. The mineral is being named brownleeite in honor of University of Washington astronomer Donald Brownlee. He is a pioneer in the study of comet dust. But, he is best known to many people as one of the authors of the book Rare Earth: Why complex life is uncommon in the universe, in which the authors make a case for Earth being pretty special. They argue that things have to be just right for higher life forms to evolve, so technologically advanced lifeforms such as ourselves are likely to be very rare in the universe.

The new mineral is believed to be from comet 26P/Grigg-Skjellerup. As comets swing around the Sun, they shed material. Some of the material is composed of ions, and it is blown away by the solar wind. But, some of the material is particulate in nature. These dust grains then often continue to orbit the Sun for a long time. In 2003, as Earth was passing through a swarm of these particles from comet Grigg-Skjellerup, NASA had a high altitude research aircraft flying. On board that aircraft was a device scooping up air and dust for a team of researchers led by Keiko Nakamura-Messenger. At that altitude, most of the dust comes from above (micrometeoroids) rather than below (volcanoes). The dust grains were analyzed, and a hitherto unknown mineral was found. This mineral was a manganese silicide (containing manganese and silicon). Brownlee had long ago proposed that some method such as this would be useful to capture cometary samples. Interestingly, he was also the principle investigator of the Stardust Mission, which sent a spacecraft to comet 81P/Wild 2. The team proposed honoring Donald Brownlee by naming the new mineral after him.

Brownleeite is particularly interesting in that it was found in space. There are fewer than 4400 minerals known. From the earliest days of space exploration, astronomers have wondered if there were minerals found elsewhere that were unlike anything on Earth. That idea was seized upon by the makers of Star Trek. In the Star Trek universe, there exists a mineral called dilithium that has special properties. Dilithium is portrayed as naturally occurring on select worlds. But, Star Trek introduced dilithium several years bore Apollo 11 returned the very first samples of rocks from another world (the Moon). But, the Moon rocks turned out to have few new minerals in them, and none with such esoteric properties as dilithium. The proportions of minerals were unlike any rocks on Earth, but almost every mineral found on the Moon had a counterpart on Earth. The unmanned spacecraft sent to Mars have similarly found minerals there that have Earth analogs. Once again, the proportions are unlike Earth rocks, but the minerals are mostly familiar. But, brownleeite is different. It is a new mineral. That makes this find exciting. Since minerals form in the way that they do as a result of their environment, this tells us that the environment under which comets form is unlike that on Earth. Of course, we were sure of that, but this is confirmation. And, since we have not found this mineral in meteorites, then it may suggest that comets even form under different conditions than the asteroids. Now, I may be reading an awful lot into just an announcement of a new mineral, but I want to underscore the significance of this find.

What makes brownleeite especially interesting is that it is not something that was expected. The press release did not go into great detail, so I don’t know much about it. But, the report did say that this mineral had not been expected to have formed in the comet or in the solar nebula. Since the mineral is obviously there, that means that scientists will have to figure out where it came from and how it got there. This suggests that we still do not understand the early solar nebula, planet formation, or the role of comets and asteroids yet.

One thing that the news report said, though, was that the mineral is a semiconductor. Being composed of nothing esoteric, the mineral can be synthesized on Earth, but it has not been found naturally occurring. Now, right off, I want to head off a few questions and people jumping to wild conclusions. The term “semiconductor” is not synonymous with the term “artificial.” In other words, semiconductivity is a material property of some materials. Everyone is well aware that some semiconductors can be manufactured, but that is about the same as saying that conductors and insulators can be manufactured. Simply because something is a semiconductor doesn’t mean that somebody made it. So, a semiconducting mineral in a comet does not imply that there were aliens messing around with the comet. Semiconductors are naturally occurring here on Earth, too. Diamonds and sapphires can also be manufactured, and they occur naturally, too. By saying that brownleeite is a semiconductor, all the mineralogists are saying is that it has the electrical property that it can be either a conductor or an insulator based upon external influences. That is what a semiconductor is. And, of course, being able to control those external influences is what semiconductor electronics is all about. Remember, one of the best known semiconductors is silicon, and that is naturally occurring on Earth (and in space).

So, brownleeite is a mystery only in that it is unknown on Earth and it is the first new mineral found in a comet, and one of the few minerals found in space that are not known here on Earth. That we didn’t expect to find it in space means that we still need work to fully understand the formation of planetary systems.

-Astroprof

Image Credit: Mary Levin, University of Washington

The debris floated away from the shuttle after it test fired its thrusters and tested its atmospheric flight control surfaces. It is not uncommon for ice to form around the engine bells at the aft end of the shuttle. And, it is not uncommon for this ice to be dislodged when the Space Shuttle moves. Also, there have been plenty of times when something in the payload bay has not been secured properly and floated out when the shuttle moved. So, in the past when things floated away from the Space Shuttle, NASA officials took notice, but were not overly concerned. However, all that has changed. On its final mission into space, an object was detected moving away from the Columbia after it had reached orbit. Some NASA engineers were quite concerned, especially after ice had been observed striking the Columbia on launch. The people calling the shots, though, didn’t take the matter all that seriously, and the result was loss of the orbiter, along with all astronauts on board. So, now, whenever something floats off from the orbiter, everyone takes note.

However, I am not implying that this is really something to worry about. After all, lots of objects have floated off during previous shuttle missions without the orbiter ever being placed in any danger. So, all that NASA engineers need to do is to determine that this is yet another event of that sort. Then, we can rest easy. But, just what was it that floated off? A clue may be in the protrusion that astronauts saw on the tail.

According to a report from Space.com, the object is now believed to be a metal clip from the area around the orbiter’s speed brake.  If that is indeed what the object is, then its loss poses no threat at all to the Space Shuttle.  These clips have come off many times before on previous missions, always without any ill effect for the Shuttle.  The clip helps to provide thermal protection to the back end of the orbiter’s tail during launch.  For a short time on the launch pad as the engines are firing up, the back end of the shuttle is exposed to quite high temperatures.  This clip is one of several devices used to protect against that heat.  It does nothing of any importance during reentry or landing, so its loss is nothing to worry about.  At present, losing the clip is only an issue because it means yet another piece of space debris is orbiting the Earth for a while.

So, what about that protrusion?  Apparently, the protrusion is believed to be an artifact of the lighting and the camera rather than an actual protrusion.  In that case, it isn’t really even there, so it is not an issue.  Another thought had been that it may be a bit of thermal insulation sticking out.  If so, it still would not have posed a threat.  It may have produced a tiny bit of drag in the area once the shuttle has slowed to regular aircraft speeds, but nothing much to worry about.  This is not an area of extreme heating during reentry.

So, in short, there is probably nothing to worry about.

-Astroprof

Images courtesy NASA TV

-Astroprof

As the name implies, Venera 4 was not the first spacecraft that the Soviet Union sent to Venus. However, it was the first one to work as expected. Contact with Veneras 1 and 2 was lost as they were en route. Venera 3 had arrived, but contact was lost before it entered the atmosphere, so it sent back no data from the most important part of its mission. Soviet scientists knew that these first spacecraft to Venus had no chance of landing, but the goal was to enter the atmosphere and send back as much data as possible before the spacecraft was destroyed.

Venera 4 worked nearly flawlessly, making it the first spacecraft to perform in situ studies of another planet’s atmosphere. On October 18, 1967, after four months traveling through space from Earth to Venus, the Venera 4 atmospheric probe made its dive into the atmosphere. It deployed a parachute to slow its descent. It then used thermometers, a barometer, and gas analyzers to monitor the atmosphere as it descended. Contact was finally lost at an altitude of about 25 kilometer.  The pressure and temperature on Venus are fatal to most spacecraft, and Venera 4 undoubtedly was crushed long before making it to the surface of the planet (though the first reports released by the Soviet Union had claimed that the craft had actually landed intact on the surface of the planet).  Even if it had not been crushed, the heat would have destroyed the electronic circuitry of the craft.

Interestingly, Venera 4 wasn’t alone in its trip to Venus.  Launched two days later, America’s Mariner 5 spacecraft also was on its way to Venus, flying by on October 19, one day after Venera 4 had arrived.

Today, with the amazing images coming back from Phoenix and the other successful space missions, we tend to overlook these important first steps in planetary exploration.  But, without spacecraft like the early Venera series or the Mariners, we wouldn’t be where we are today in planetary exploration.

-Astroprof

Image Credit:  NASA, NSSDC